(1) Field of the Invention
The invention relates to field emission flat panel displays, and more particularly to methods for fabricating high aspect ratio spacers for such displays.
(2) Description of the Related Art
In display technology, there is an increasing need for flat, thin, lightweight displays to replace the traditional cathode ray tube (CRT) device. One of several technologies that provide this capability is field emission displays (FED). An array of very small, conical emitters is manufactured, typically on a semiconductor substrate as part of a base plate, and is addressed via a matrix of columns and rows of conductive lines connected to peripheral addressing logic circuits. The emitters are formed on a conductive cathode, and surrounded by another conductive layer typically called the gate. When proper voltages are applied to the cathode and gate, electrons are emitted and attracted to an anode which is placed on a face plate opposite to the base plate. A pattern of cathodoluminescent material on the anode emits light when excited by the emitted electrons, providing the display element.
The base and face plates are mounted in very close proximity, in order to form a thin display and to maintain a high display resolution. A vacuum is formed in the area between the two plates, typically less than 10.sup.-6 torr. It is important to maintain a uniform spacing between the opposing plates in order to provide consistent resolution across the display surface. In order to maintain this uniform spacing in the presence of the vacuum, spacers are typically placed between the opposing plates. Except for displays with a very small surface area, e.g., on the order of a few square inches or less, these spacers are required, in order to maintain consistent spacing in light of the large pressure differential between the outside of the face plate and the evacuated region.
Other requirements of spacers for a field emission display include a small cross-sectional area and proper registration. The spacers must be small enough in cross-section to prevent being visible to a viewer of the display, so a high aspect ratio is necessary. The process for forming the spacers, and integrating them with the display face and base plates, should provide a simple means for aligning the emitting surface with the opposing face plate.
Workers in the field are aware of these problems and have attempted to solve them. In U.S. Pat. No. 4,923,421, Brodie et al. disclose a method of forming spacers using polyimide resins as the spacer material, and standard photolithographic techniques to form the spacers. However, the use of polyimide can cause problems due to outgassing, i.e., the release of volatile components, with the problems including poor electron emission and short emitter life.
In U.S. Pat. No. 5,232,549 (Cathey et al), after forming a polymer material from which the spacers will be formed, and a thin patterned reflective layer above the polymer, a laser is used to ablate away material after which the spacers remain. Alternately, a laser is used to form holes in an etchable layer, and the holes are filled with a spacer material, after which the etchable material is removed.
U.S. Pat. No. 5,205,770 (Lowrey et al) discloses a similar process in which a micro-saw is used to form grooves in a substrate, the grooves are filled with a spacer material, chemical mechanical polishing is performed on both ends of the spacers, and the mold is removed. A drawback with this technique is the requirement of an additional frit seal to connect the spacers with both the baseplate and faceplate.
None of the preceding methods, however, address the problem of alleviating charge build-up on the spacers. During display operation, electric charge may accumulate on the spacers, and if not discharged in a controlled means result in disturbance of the screen image.